The present invention relates to cooling apparatus and, particularly, to a cooling apparatus in which liquified gas is introduced into a chamber and cold vapor is extracted from the chamber.
Differential thermal analysis (DTA) generally refers to a calorimetric technique for measuring physical properties of a substance by exposing the substance to different temperature regimes. DTA can be employed to measure parameters associated with phase transitions, glass transitions, polymerization/depolymerization, crystallization, softening, sublimation, dehydration, decomposition, oxidation, cure kinetics and so forth. A differential scanning calorimeter (DSC) measures temperatures and heat flows associated with energy-emitting or energy-absorbing (exothermic and endothermic, respectively) material transitions. DSCs are widely used in academic, government and private facilities for research purposes, as well as for quality control and production purposes.
Hereinafter, reference will be made to DSC, although it is to be understood to encompass DTA as well.
During DSC testing, the material being analyzed (xe2x80x9csamplexe2x80x9d) is heated or cooled according to a desired temperature profile. The results, such as differential temperature or heat flow, are measured and analyzed to understand the properties of the sample material. The basic theory of DSC analysis is well understood; the reader is referred to Reading, et al., U.S. Pat. No. 5,224,775 (the ""775 patent) and U.S. Pat. No. 3,456,490 (the ""490 patent) for details on the theory of operation of exemplary DSC systems. The ""775 and ""490 patents are herein incorporated by reference in their entirety.
There are also other well-known thermal analysis techniques, such as Pressure Differential Scanning Calorimetry (PDSC), Pressure Differential Thermal Analysis (PDTA), and Differential Photocalorimetry (DPC). The invention described hereafter may also be applied to such instruments.
Typical DSC instrumentation includes the following basic components: a heated measurement chamber enclosing a sensor assembly upon which the material to be analyzed is placed; a furnace heater for heating the measurement chamber; and a cooling device for cooling the measurement chamber.
The cooling device may find application when temperature is being increased or decreased. When temperature is being increased, the cooling device may act as a heat sink for the furnace heater. For example, during above-ambient operations at 400xc2x0 C., heat generated by the furnace heater will be channeled to the cooling device for dissipation, providing a stable load to control the heater against.
When temperature is being decreased, e.g., for analysis at below-ambient temperatures, the cooling device is used to drive the measurement chamber down to the desired temperature. For example, the cooling device may be used to cool the measurement chamber down to xe2x88x92180xc2x0 C.
Cooling devices used with DSC instrumentation include various types of heat exchangers, such as gas-cooled heat exchangers, liquid-cooled heat exchangers, and change of phase liquid-gas heat exchangers.
Gas-cooled heat exchangers rely on the cooling effect of a gas removing heat from the heat exchanger. Typically, gas-cooled heat exchangers employ vaporized nitrogen as the cooling agent for sub-ambient operation. Gas-cooled heat exchangers, however, suffer several significant drawbacks. First, if liquid nitrogen is vaporized to generate the cold gas, most of the cooling power of the liquid is lost in converting it to a gas. Second, gas is very inefficient at removing heat due to its low heat capacity and high thermal resistance.
Liquid-cooled heat exchangers rely on the cooling effect of a liquid circulating in the heat exchanger. Typically, liquid-cooled heat exchangers employ water, freon or possibly, ethylene glycol, as the cooling agent. Liquid-cooled heat exchangers, however, suffer several significant drawbacks. First, cooling the liquid requires an additional heat exchanger stage to keep the liquid cool as it removes heat from the DSC. Thus, the cooling provided by a liquid cooled heat exchanger is still significantly less efficient than a change of phase liquid-gas system. Second, because the liquid is constantly circulated, contamination of the liquid can result in poor performance and clogging of the circulation tubing. Also, because water is often used, liquid-cooled heat exchangers do not cool very effectively to sub-ambient temperatures.
Change of phase liquid-gas systems are desirable because they rely on the endothermic (energy absorbing) nature of the heat of vaporization. Because the heat exchanger""s interaction with the liquified element results in vaporization of the element, a greater amount of heat energy is removed from the heat exchanger. Thus, a nitrogen-based change of phase system provides significantly more cooling than a similar nitrogen-based gas cooling system.
However, current change of phase liquid-gas systems suffer significant drawbacks that limit their practicality. For example, the amount of nitrogen supplied may exceed that which can be vaporized. This results in liquid in the exhaust, which is generally undesirable, and which can lead to frost, leakage, and overflow. On the other hand, if the flow of nitrogen is restricted to substantially eliminate the incidence of liquid in the exhaust, performance in terms of maximum cooling rate and minimum temperature may be unnecessarily compromised. In general, designs for change of phase liquid-gas systems have not permitted realization of the full potential of this approach to cooling. This is a significant drawback.
Additionally, a heater control system will be adversely impacted if the energy removed by the heat exchanger changes rapidly as would occur if the liquid level in the heat exchanger fell to the point where a layer of gas formed between the liquid and the surface of the heat exchanger. This is a significant drawback.
To overcome these drawbacks or disadvantages in the prior art, and in accordance with the purpose of the invention, as embodied and broadly described, an embodiment of the present invention comprises a nitrogen-based change of phase liquid-gas cooling system including a heat exchanger, a liquid detection/evaporator assembly, a liquid detection feedback loop, and a pressure control device.
A pressure-controlled supply reservoir (e.g., a dewar) feeds a cooling agent such as liquid nitrogen to a heat exchanger mounted to a DSC cell. The DSC cell is cooled as liquid nitrogen contacting the surface of the heat exchanger is vaporized into nitrogen gas. The exhaust (nitrogen gas and, occasionally, small amounts of nitrogen liquid) is fed to a liquid detection/evaporator assembly. If liquid nitrogen in the exhaust is detected by the liquid detection/evaporator assembly, an indication is fed back to a pressure control device using a liquid detection feedback loop. The pressure control device adjusts the amount of pressure on the nitrogen source in order to eliminate liquid in the exhaust. During the cycle where there is liquid in the exhaust, the liquid detection/evaporator assembly also collects and vaporizes the liquid in the exhaust stream so that it can be properly vented to atmosphere in gas form.
The advantages of the present change of phase liquid-gas cooling system are numerous. The liquid detection feature enables a feedback control capability for providing an amount of liquid nitrogen that maximizes cooling while minimizing liquid in the exhaust. The evaporator feature provides for any residual liquid in the exhaust to be vaporized before release, thus reducing frost, leakage, overflow and other problems. Overall, the present invention permits liquid-gas heat exchange to be used to its optimum potential as a most effective cooling system, while minimizing the problems which otherwise might make it impractical.
Accordingly, an object of the invention is to provide a liquid-gas cooling system including a liquid detection means for detecting the presence of a liquid cooling agent, such as liquid nitrogen, in the cooling system exhaust.
Another object of the invention is to provide a liquid detection feedback loop so that, upon the detection of liquid in the exhaust, the amount or level of cooling agent can be adjusted to reduce or eliminate further liquid in the exhaust.
Another object of the invention is to provide a liquid-gas cooling system having evaporator means for vaporizing liquid cooling agent found in the exhaust prior to its release.